A magnetic semiconductor compound is a unique compound having both semiconducting electrical property and magnetic property. Because the magnetic semiconductor compound exhibits a large magnetoresistive effect near the magnetic transition temperature (Curie temperature Tc or Neel temperature TN), its application to magnetic sensors and the like are expected. However, most magnetic semiconductor compounds discovered so far exhibit only one conductivity type, i.e., either n-type or p-type, and no magnetic semiconductor compound that can exhibit both conductivity types, i.e., n-type and p-type, has been discovered to date except for CdCr2Se4 (non-patent document 1).
In CdCr2Se4, the 3d6 electron of a Cr3+ ion is accountable for magnetic property and 4p of Se and the 4s electron of Cd are the hole and the electron accountable for the conduction. Thus, there is little interaction between the magnetic property and the electrical conduction. Furthermore, Cr3+ having a 3d6 electron configuration disadvantageously exhibits a magnetic moment smaller than and a Curie temperature (150 K) lower than those of the 3d5 electron configuration of the Mn2+ ion and the Fe3+ ion. Due to these drawbacks, a pn homojunction magnetic device suitable for practical use has not been available.
Another approach of making various magnetic devices is by using a pn junction structure of a magnetic semiconductor and a nonmagnetic semiconductor (nonpatent documents 2 to 4). These devices are named spin bipolar devices but are not put to practical use because formation of high-quality pn junction structures is difficult and the magnetic transition temperature of the magnetic semiconductor is lower than room temperature.    Nonpatent document 1: P. W. Cheng et al. IEEE, Trans. Magn. 4, 702-704 (1968)    Nonpatent document 2: M. Johnsonn et al. Phys. Rev. Lett. 55, 1790 (1985)    Nonpatent document 3: Fiederling et al. Nature 402, 787 (1999)    Nonpatent document 4: Ohno et al. Nature 402, 790 (1999)